How to insert a cut here line?

Use a line shape from the Shape icon in the tool bar. If you want to rotate it you can hold down the cmd key while dragging one handle on the selected line

You can change the thickness in the Inspector palette > Graphic tab.

To add to fruhulda's suggestion you might want to make it a dotted line (in the drop down list above 'Endpoints' in the picture above).

You could also add some scissors (try the Wingdings fonts).

Adding to what fruhulda says...

The 'line' button at the left end of the Format bar also found in the Stroke section of the Inspector pane in fruhulda's message) is a pop-up menu offering several choices of line type:

At the bottom of sample lines list to the right, a text box containing a 14 pt 'scissors' character has been grouped with a 1.5 pt version of the the dashed line.

You can produce a similar scissors & dashed line combination with text, using a scissors character followed by a tab, the tab formatted to include a dashed leader:

Regards,

Barry